algorithm-class/体系学习班/class01/Code06_BSAwesome.java

package class01;

public class Code06_BSAwesome {

	// 课上的代码
	public static int getLessIndex(int[] arr) {
		if (arr == null || arr.length == 0) {
			return -1;
		}
		if (arr.length == 1 || arr[0] < arr[1]) {
			return 0;
		}
		if (arr[arr.length - 1] < arr[arr.length - 2]) {
			return arr.length - 1;
		}
		int left = 1;
		int right = arr.length - 2;
		int mid = 0;
		while (left < right) {
			mid = (left + right) / 2;
			if (arr[mid] > arr[mid - 1]) {
				right = mid - 1;
			} else if (arr[mid] > arr[mid + 1]) {
				left = mid + 1;
			} else {
				return mid;
			}
		}
		return left;
	}

	// 验证得到的结果，是不是局部最小
	public static boolean isRight(int[] arr, int index) {
		if (arr.length <= 1) {
			return true;
		}
		if (index == 0) {
			return arr[index] < arr[index + 1];
		}
		if (index == arr.length - 1) {
			return arr[index] < arr[index - 1];
		}
		return arr[index] < arr[index - 1] && arr[index] < arr[index + 1];
	}

	// 为了测试
	// 生成相邻不相等的数组
	public static int[] generateRandomArray(int maxSize, int maxValue) {
		int[] arr = new int[(int) (Math.random() * maxSize) + 1];
		arr[0] = (int) (Math.random() * maxValue) - (int) (Math.random() * maxValue);
		for (int i = 1; i < arr.length; i++) {
			do {
				arr[i] = (int) (Math.random() * maxValue) - (int) (Math.random() * maxValue);
			} while (arr[i] == arr[i - 1]);
		}
		return arr;
	}

	// 为了测试
	public static void main(String[] args) {
		int testTime = 500000;
		int maxSize = 30;
		int maxValue = 100;
		System.out.println("测试开始");
		for (int i = 0; i < testTime; i++) {
			int[] arr = generateRandomArray(maxSize, maxValue);
			int ans = getLessIndex(arr);
			if (!isRight(arr, ans)) {
				System.out.println("出错了！");
				break;
			}
		}
		System.out.println("测试结束");
	}

}